1. Field of the Invention
The present invention relates to an image compression/decoding apparatus and method, and more particularly, to an image compression/decoding apparatus and method for wavelet-transforming and quantizing digital image data and compressing/decoding quantized data.
2. Description of the Related Art
A purpose of compressing digital image data is to reduce a bit rate of an imput image or enhance an efficiency of a storage unit for image data.
There are a block DCT (Discrete Cosine Transform) coding method and a wavelet transform coding method as an image data compression method. The block DCT coding method has problems that a blocking effect and mosquito noise occur. The wavelet transform coding method can overcome the above problems.
FIG. 1 is a block diagram showing a conventional image compression apparatus using wavelet transform.
The conventional image compression apparatus using wavelet transform of FIG. 1 includes a wavelet transformer 1 for wavelet-transforming received digital image data ID, a quantizer 2 for dividing the wavelet transformed data by a bin size and outputting quantization coefficient data having a range between −255 and +255, and a Huffman encoder 3 for compressing the quantization coefficient data by use of a Huffman coding method and outputting compressed data CD.
FIG. 2 is a transform state diagram showing the state of an image divided by wavelet transform in a wavelet transformer 1, in which digital image data is divided once.
As shown in FIG. 2, the wavelet transformer 1 passes digital image data through a vertical low pass filter and a vertical high pass filter, and passes the output of the vertical low pass filter through a horizontal low pass filter and a horizontal high pass filter, to thereby output a first wavelet transformed image LL and a second wavelet transformed image LH. Also, the wavelet transformer 1 passes the output of the vertical high pass filter through the horizontal low pass filter and the horizontal high pass filter, to thereby output a third wavelet transformed image HL and a fourth wavelet transformed image HH.
In addition, the first wavelet transformed image LL can be divided again through the above process. The Huffman encoder 3 forms a Huffman tree according to the number of frequency in quantization coefficient data of each pixel in which data of each pixel in the first through fourth wavelet transformed images LL, LH, HL and HH divided in FIG. 2 is quantized. Accordingly, quantization coefficient data having the highest frequency number is coded into the smallest number of bits, and quantization coefficient data having the lowest frequency number is coded into the largest number of bits, to thereby output final compressed data.
The image data compression apparatus for compressing digital image data by using a conventional Huffman encoder uses hardware and software to form a Huffman tree with respect to all quantization coefficient data obtained by quantizing each wavelet transformed image, to thereby perform a Huffman coding. Accordingly, the amount of calculation increases and it takes much time to perform the Huffman coding. Also, a circuit for embodying the digital image data compression apparatus becomes complicated.
Also, although quantization coefficient data obtained by quantizing the first wavelet transformed image LL of a low frequency region in the quantizer 2 is distributed in various forms and quantization coefficient data of a high frequency region other than the first wavelet transformed image LL region is often distributed with the small number of quantization coefficient data, the conventional image data compression apparatus performs the Huffman coding identically with respect to all the quantization coefficient data irrespective of the various and particular distribution of the quantization coefficient data. As a result, inefficient data compression may be caused and a memory device having a large storage capacity should be used in order to store compressed data.